Energy Materials 2017: Materials for Gas Turbines: Microstructure and Processing
Sponsored by: Chinese Society for Metals
Program Organizers: Jeffrey Fergus, Auburn University; Ji Zhang, China Iron and Steel Research Institute Group

Tuesday 2:00 PM
February 28, 2017
Room: 13
Location: San Diego Convention Ctr

Session Chair: Jeffrey Fergus, Auburn University

2:00 PM  Invited
Modeling the Diffusion of Minor Elements in Different MCrAlY – Superalloy Substrates at High Temperature: Krishna Jonnalagadda1; Kang Yuan2; Xin-Hai Li3; Ru Peng1; Yueguang Yu2; 1Linkoping University; 2Beijing General Research Institute of Mining and Metallurgy; 3Siemens Industrial Turbomachinery
    As demands of energy supply have been increased continuously and at the same time uses of fossil fuel are limited and the greenhouse effect should be minimised, Siemens has made great efforts to increase gas turbine efficiency and to reduce gas turbine emissions for power generations. One of the efforts is a continuous development of high temperature capacity of ceramic thermal barrier coatings (TBCs) and metallic MCrAlY overlays. The MCrAlY overlays are used as both protective coatings or bondcoats to TBCs on underlying superalloy components in the gas turbines. During high temperature exposure, elemental diffusion occurs between the bond coat and the substrate which can affect the overall coating performance. The present study investigates the diffusion of minor elements like Re, Ta, Si, Mo, W, Ti, and Nb in various MCrAlY overlays and superalloy substrates. An oxidation-diffusion model has been used to study the elemental diffusion. The diffusion process contains two stages: β depletion stage and the β depleted stage. In the stage when MCrAlY overlays exhibit γ+β microstructure, the diffusion of minor elements in the coatings was observed to be related to the β depletion rate. After that the diffusion of the minor elements were observed to be independent of the coating composition. The development principle of the elemental diffusion is discussed in detail.

2:30 PM  
On Healing Mechanism of Cast Porosities in Cast Ni-Based Superalloy by Hot Isostatic Pressing: Yuan Chao1; Li Jie1; Kai-Xin Dong1; Guo Jianting1; 1Institute of Metal Research, Chinese Academy os Sciences
    The purpose of this study is to evaluate the effects of hot isostatic pressing (HIP) on the porosity healing and the microstructure evolution of a cast superalloy K452, which is generally applied for fabrication the vanes with the bigger size. The annihilation degree of the porosity, related to the location and shape of the porosity as well as HIP temperature, were studied by high resolution transmission X-ray tomography (HRTXRT) and finite element method (FEM). Results showed that the most of porosities (over 80 %) were eliminated by HIP at 1200 oC, in which the porosity located in the thin walled parts and with the loose-flat shape was easy to be healed, and the diameter of remnant porosities was decreased to less than 10 m. Accordingly, A healing mode of the porosity was established. .Furthermore, It was found that the slow cooling rate during HIP would obviously lead γ′ precipitates to coarsen, while the rejuvenation heat-treatment (RHT) after HIP could reduce the dendritic segregation degree and readjust the coarsen γ′ precipitates, which finally improved the mechanical properties of the cast superalloy. The HIPed K452 alloy vanes have been successfully used in a civil gas turbine.

2:50 PM  
Simulation of Precipitation Behavior of Nickel-based Superalloys: Fan Zhang1; Weisheng Cao1; Shuanglin Chen1; Chuan Zhang1; Jun Zhu1; 1CompuTherm, LLC
    Precipitation hardening provides one of the most widely used mechanisms for the strengthening of nickel-based superalloys. Modeling of precipitation process plays an important role in understanding the temporal evolution of microstructure and the corresponding responses of mechanical properties of nickel-based superalloys. However, precipitation is a complex process involving the simultaneous occurrence of nucleation, growth and coarsening. Accurate modeling of the precipitation process requires a synchronous consideration of all these contributions. Moreover, the phase equilibrium information as well as the mobility data of matrix phase needs to be constantly updated as the nucleation, growth and coarsening proceed. Such a simulation, therefore, necessitates a smooth integration of thermodynamic calculation, kinetic simulation and property modeling of the material. In this Presentation, we will present the simulation tool we have developed in this regard, and commercial nickel-based superalloys will be used as examples to demonstrate the applications of this modeling tool.

3:10 PM  
Microstructures and Mechanical Properties of Ultrafine Grained Ni Based Superalloy Matrix Nanocomposites Fabricated by Powder Metallurgy Route: Tian Xia1; Deliang Zhang2; Jiantao Liu3; Yiwen Zhang3; 1Shanghai Jiao Tong University, China; 2Northeastern University, China; 3Central Iron and Steel Research Institute
    Ultrafine grained Ni based superalloy matrix nanocomposites reinforced by Y2O3 nanoparticles were fabricated by high energy mechanical milling of Ni base superalloy machining chips or powders together with Y2O3 nanoparticles followed by thermomechanical powder consolidation through powder compact sintering and hot extrusion. The purpose of this study is to develop new type of high temperature materials based on ceramic nanoparticle stabilization of ultrafine grain structure and grain boundary strengthening. The microstructures of the consolidated samples were characterized and their tensile mechanical properties and fracture behavior at room and elevated temperatures were measured and studied. The contributions of grain boundary strengthening, nanoparticle strengthening and other strengthening mechanisms such as solid solution strengthening and gamma prime phase strengthening are analyzed. The roles of inperparticle boundaries in controlling the room and elevated temperature ductility of the consolidated material are discussed.

3:30 PM Break

3:50 PM  
Rejuvenation of a Co Based Superalloy to Prevent the Quickest Microstructural Degradation during the Following Operating Cycle: Erica Vacchieri1; Giacomo Roncallo2; Gabriele Cacciamani2; Alessio Costa2; 1Ansaldo Sviluppo Energia S.p.A.; 2Chemistry Department, University of Genoa
    The rejuvenation of an old generation Co based superalloys was developed to recover the alloy starting microstructure and to prevent its quicker degradation during the following service cycle of a GT first stage vane. An in-house thermodynamic database was developed to predict the martensitic fcc-hcp transformation of the alloy matrix during service and it was validated through experimental tests. The dissolution of hcp phase happens at temperature above 800C but the martensitic transformation leaves some sub-grain microstructure that improves the hcp precipitation during the following service cycle. Several operated components were considered and a rejuvenation process was designed in order to remove the sub-structure from the alloy and prevent the quicker transformation during the following operating period.

4:10 PM  
Rejuvenation Process Definition for IN792SX Gas Turbine Blades Aimed to Extend Their Expected Life: Erica Vacchieri1; Paola Guarnone1; Elena Bergaglio1; 1Ansaldo Sviluppo Energia S.p.A.
    The energy market requests drive the GT-OEM to strongly flexible operative regimes and cost reduction maintenance plans. In this perspective, the component life extension is the main target. The extend of maintenance intervals and the number of permissible life cycles are related to the possibility to rejuvenate the HGP critical component microstructure. The rejuvenation for Ni based single crystal superalloy blade is a critical process because recrystallization can occur and the SX alloy chemical composition does not provide grain boundary strengthening, responsible for instantaneous cracks during the following service cycle. In the present paper an alternative rejuvenation process was defined for IN792SX blades, considering components after service and different operating regimes. The microstructural investigations and the quantitative measurements collected through EBSD and DTA allows the definition of the heat treatment plan based on the component condition after service. Quantitative measurements of material recovery was evaluated through mechanical tests.

4:30 PM  
Tensile Behavior of Inconel X-750: Effect of Heat Treatment: Christopher Marsh1; Djamel Kaoumi2; 1University of South Carolina; 2North Carolina State University
    X-750 is a nickel-chromium based super alloy of usefulness in a wide variety of applications such as gas turbines, rocket engines, nuclear reactors, etc. Its good mechanical properties are due to the strengthening from precipitation of γ’ particles upon ageing heat treatment. The mechanical properties of both heat treated and as received (solution annealed only) X-750 have been tested by uniaxial tensile testing under a large temperature range, room temperature to 900C, and at three strain rates (10^-3, 10^-4, 10^-5). The mechanical property progression due to temperature and heat treatment variation, as well as the plastic behavior were investigated. Dynamic strain aging and the PLC (Portevin–Le Chatelier) effect have been frequently observed in nickel base superalloys; the PLC regime, depending on strain rate and temperature, was studied in X-750. Through observational techniques including TEM and SEM, material characterization was utilized to explain mechanical behavior and microstructural evolution.

4:50 PM  
The Influence of Dendritic Segregation Degree to the Recrystallization Nucleation in U4720LI: Jiayu Chen1; Jianxin Dong1; 1University of Science and Technology Beijing
    The effect of dendritic segregation degree on volume fractions and nucleation sites of recrystallization was investigated through comparing three kinds of samples with heavy degree of dendritic segregation, with light degree of dendritic segregation and without dendritic segregation. The degree of dendritic segregation was controlled by altering homogenization time. Stress concentration was caused by uneven distributions of γ’ phases in dendrite arm and interdendritic areas. Meanwhile, recrystallization was retarded due to the pinning effect of γ’ phases in interdendritic areas. Thus, recrystallization nucleation started along the grain boundary, then in the junction of dendrite arm and interdendritic areas but at the side of dendrite arm areas with the increase of deformation degree. Finally, nucleation occurred in the interdendritic area. Moreover, strain concentration was released as γ’ phases were more evenly distributed with the increase of homogenization time which lead recrystallization volume fractions reduced with the decrease of dendritic segregation.

5:10 PM  
Grain Refinement of Cast FeAl-Alloys for Gas Turbine Blades: Heiner Michels1; Thomas Brenker2; Laura Klinkenberg3; Matthias Buenck1; 1Access e.V.; 2Other; 3RWTH Aachen University
    Flexible power plants are required to compensate fluctuations in an electricity grid primarily based on sustainable energy. Gas turbines offer this flexibility, namely by short start-up times, their suitability for frequent load changes and an overall efficiency of around 60%. The blades in the back turbine stages are often made from high alloyed chromium steels suitable for operating temperatures at around 620 C. Due to the high costs of these alloys, alternative materials are in demand. FeAl intermetallics are suitable for the said temperatures, of lower density and available at lower cost. However, the low ductility at room temperature is a significant problem. The current work presents results of a study aiming at increasing the mechanical and casting properties of FeAl-cast parts by means of grain refining FeAl-alloys. The study comprises tests of grain refiners and refining methods as well as analytical results of the achieved microstructures.